Optical absorption gas sensors determine the concentration of a gaseous analyte in a gas sample by measuring the attenuation of light passing through the gas sample due to absorption by the analyte of interest. Many such sensors use light in the infra-red region where gases such as carbon dioxide and methane have absorption lines.
In dispersive infra-red sensors, light of a series of wavelengths which includes one or more absorption lines of the analyte which is to be detected is directed through a gas sample and its intensity is measured by a detector. In non-dispersive infra-red (NDIR) sensors, light of a broad range of wavelengths is passed through a gas sample and the intensity of light in a selected wavelength band, which includes one or more absorption lines of the analyte of interest, is measured, typically using a band-pass filter and a light detector, such as a thermopile, photodiode or pyroelectric detector.
In optical absorption gas sensors, the concentration of the analyte gas can be established using the Beer-Lambert Law; Il=Io10−αLc where Io is the intensity of light that is incident on a gas sample, Il is the intensity of light after passing through the material, L is the distance that the light travels through the material from the source to the detector (the path length), c is the concentration of the analyte in the material, and α is the absorption coefficient of the analyte species. The Beer-Lambert law applies to monochromatic radiation and the corresponding relationship for polychromatic light can be established by summation/integration. In some optical absorption sensors, light reaching the detector will have components with a range of path lengths, and so there is usually some apparent deviation in practice from the theoretical predictions of the Beer-Lambert Law which can be determined from the geometry of the sensor and empirical measurements.
Many optical absorption gas sensors measure the intensity of light which has passed through the gas sample at two different wavelength bands, one of which includes absorption lines of the analyte, and one of which does not and so functions as a reference. For example, in an NDIR sensor, it is known to use a single light detector which can measure light at two different wavelengths, perhaps using a variable-wavelength Fabry-Perot interferometer, or to use separate measurement and reference light detectors, each of which has a different wavelength band-pass filter.
In order to improve the precision and sensitivity of optical absorption gas sensors, it is desirable to maximise the path length of the light which falls on the detector and therefore increase the total absorption due to a given concentration of analyte. For example, U.S. Pat. No. 4,618,771 (Beckman Industrial Corporation) and U.S. Pat. No. 5,850,354 (Vaisala Oy) disclose elongate analysers in which the path length is maximised by using a long and relatively thin measurement chamber.
However, there are many industrial applications where it is desirable to minimise the size of a sensor. This may be to reduce bulk, or to improve the speed of response of the sensor, by minimising the volume of the measurement chamber and therefore the amount of time required to introduce a gas sample to the measurement chamber whether actively (e.g. using a pump) or passively by diffusion. A number of solutions have been proposed in the art for maximising the path length for a given size of sensor. Typically, these solutions provide sensors in which light from an appropriate source is reflected many times through the gas sample before it reaches the detector. Examples of sensors of this type are disclosed in GB 2,391,310 A (Edinburgh Instruments Limited), GB 2,395,260 A (e2v Technologies Limited), and WO 02/063283 (Dynament Limited).
A disadvantage of sensors in which light is reflected many times is that small manufacturing errors can have a significant adverse effect on the amount of light reaching the detector. Imperfections may amplify variations in the amount of light which reaches the detector due to variations in the intensity or exact position of the light source in use. These problems are particularly acute for sensors which comprise separate measurement and reference light detectors. In this case, errors which affect the relative amount of light which reaches the measurement and reference light detectors, or which affect the relative distribution of path lengths of light reaching the measurement and reference detectors, are of particular concern. Therefore, an important design parameter, which is taken into account in the present invention, is the tolerance of the sensor to manufacturing imperfections and variations in the intensity, spectral properties and/or path of light in use.
These issues are particularly critical in sensors which must occupy small volumes. For example, in the gas sensing industry, there is a standard gas sensor configuration, which takes the form of a cylinder with a 20 mm diameter, and a height of 16.6 mm. This is a confined volume which presents a significant engineering challenge. Furthermore, commonly packaged light-emitting devices and detectors are of a size which can become a significant proportion of the volume of the sample chamber in smaller sensors.
Some aspects of the present invention aim to provide an optical absorption gas sensor which balances the above factors to provide a good performance relative to other sensors of similar size and manufacturing cost.